1. Field of the Invention
The present invention relates to an oscillating device, particularly to a compact oscillating device that has an excellent temperature characteristic and can be produced in large quantity at a low price.
2. Description of the Related Art
A temperature compensated crystal oscillator (TCXO) in which a crystal oscillation circuit and a temperature compensation circuit are combined can be cited as an example of an oscillator oscillated at a stable frequency against a temperature change.
Recently a compact communication system such as worldwide interoperability for microwave access (WiMAX) and a femto cell is actively developed.
Frequently, a temperature characteristic better than that of a normal TCXO is required in such applications.
For example, a general TCXO has a temperature characteristic of about ±200 ppb (operating temperature range of −20° C. to +70° C.). On the other hand, the required temperature characteristic ranges from about −50 ppb to about +50 ppb (operating temperature range of −20° C. to +70° C.). Therefore, it is difficult that the required temperature characteristic is realized by the general TCXO.
An oven controlled crystal oscillator (OCXO) including a constant-temperature oven is well known as the oscillating device having the temperature characteristic better than that of the TCXO. However, disadvantageously, the OCXO is more expensive and larger than the TCXO. Therefore, there is the need of a compact, low-price oscillating device having a temperature characteristic intermediate between the TCXO and the OCXO.
[Configuration of Conventional Oscillating Device]
Conventionally, there is an oscillating device in which a temperature of the TCXO is kept constant to improve the temperature characteristic of the frequency compared with the single TCXO.
A configuration of the conventional oscillating device in which the temperature of the TCXO is kept constant will be described with reference to FIG. 3. FIG. 3 is an explanatory view schematically illustrating a configuration of a conventional oscillating device, FIG. 3A is a side view of the conventional oscillating device, and FIG. 3B is a plan view of the conventional oscillating device.
As illustrated in FIG. 3, the conventional oscillating device has the configuration in which a TCXO 12, a power transistor 13, a temperature controlling OP amplifier 14, and a heating resistor 15 are mounted on a glass epoxy board 11.
A board 11 on which the TCXO 12 and various electronic components are mounted is disposed on a base, and a cover is provided on the base to seal the base.
In the conventional oscillating device, the number of components except for the TCXO is increased, and the components are disposed around the TCXO 12. Therefore, it is difficult to reduce a plane area of the whole of oscillating device. Thus, a volume of the oscillating device is much larger than that of the single TCXO (10 times or more), and the oscillating device becomes expensive.
[Document of Related Art]
For example, conventional techniques regarding the structure of the oscillating device are disclosed in Japanese Patent Application Laid-Open Nos. 2005-223395 (applicant: TOYO Communication Equipment Co., Ltd), 2007-201616 (applicant: Epson Toyocom Corporation), and 2007-235482 (applicant: KYOCERA KINSEKI Corporation).
Japanese Patent Application Laid-Open No. 2005-223395 discloses the oscillator having the following structure. In a TCXO, a board on which an integrated circuit is mounted is provided on an integrated circuit, a temperature control circuit and a heating element are incorporated in the integrated circuit, and a sub-board on which an oscillating device and an oscillating circuit are mounted is connected to the board.
Japanese Patent Application Laid-Open No. 2007-201616 discloses the oscillator having the structure, in which IC including a temperature compensation circuit and an oscillating circuit is surrounded by a flexible board and a piezoelectric oscillator package is mounted on the IC.
In the oscillator disclosed in Japanese Patent Application Laid-Open No. 2007-235482, a first space and a second space are formed across a board from each other, a piezoelectric oscillating piece is accommodated in the first space, and IC is accommodated in the second space. A temperature sensor, a temperature compensation circuit, and an oscillating circuit are incorporated in the IC.
In the conventional oscillating device of FIG. 3 in which the temperature of the TCXO is kept constant, although the temperature characteristic of the frequency is improved compared with the single TCXO, unfortunately the TCXO is not efficiently heated, the number of components is increased to hardly achieve the miniaturization, and the oscillating device becomes expensive.